1. Field of the Invention
This invention relates to a device for converting a frame rate in a moving picture decoder which outputs stream data, whose output frame rate has been set to 24 Hz, at an output frame rate of 30 Hz or 60 Hz.
2. Description of the Related Art
The present MPEG (Moving Picture coding Experts Group) standard defines an input stream whose frame rate, which represents a period of displaying one frame of a picture, is set to 24 Hz, 30 Hz or 60 Hz, for example. When the rate of frames contained in the stream data is 30 Hz, a television set which is provided with a moving picture decoder and to which a frame rate of 30 Hz has been set decodes the encoded input stream in accordance with this frame rate thereby to display pictures.
When stream data having a frame rate of 24 Hz is inputted to a television set which is provided with a moving picture decoder and whose frame rate has been set to 30 Hz or 60 Hz, the frame rate of 24 Hz must be converted to 30 Hz or 60 Hz in order to display pictures.
For such a reason, when decoding stream data whose frame rate has been set to 24 Hz thereby to display pictures, the conventional moving picture decoder causes one frame of a picture to be re-displayed (i.e., one frame to be repeated) once every four frames thereby to bring the display period of the pictures into agreement with a period which corresponds to the frame rate set thereto. In addition, the conventional moving picture decoder may cause such an operation that the frame rate is set erroneously to 24 Hz due, for example, to an error occurring during the encoding, even when a re-display of picture is indicated by the Repeat_First_Field flag, with the result that one frame is repeated once every four frames to display pictures.
The conventional moving picture decoder thus repeats one frame of pictures once every four frames, so that the quality of pictures is low because the displayed pictures are rather coarse and it is difficult to attain a smooth display of pictures.
It is therefore an object of the present invention to provide a frame-rate converting device for a moving picture decoder which makes it possible to perform a smooth display of pictures even when a display is made on a monitor of 30 Hz or 60 Hz based on stream data whose input frame rate is 24 Hz.
In order to achieve the above object, according to the present invention a repeat-first-field information, a progressive-sequence information and a progressive-frame information, which correspond to each picture, are extracted from a received stream as header information and a control is performed such that pictures are displayed in accordance with the extracted header information. Thus, pictures can be displayed in a smooth manner according to the invention.
The present invention may be arranged to determine whether each picture is of the progressive scanning based on the progressive-sequence information when the repeat-first-field information contained in the received header information does not indicate a re-display of picture, and to re-display one field of a picture once every two frames when the determination result indicates the progressive scanning. In this way, the pictures can be displayed in a smooth manner.
The present invention may also be arranged to determine whether each picture is of the progressive scanning based on the progressive-sequence information, to further determine whether a first field and a second field constituting one frame of a picture are composed of picture data of the same time instant (i.e., whether the picture has a frame structure of the progressive-scanning type) based on the progressive-frame information when the result of the former determination does not indicate the progressive scanning, and to re-display one field of a picture once every two frames when the latter determination indicates that the picture data is of the same time instant. In this way, the pictures can be displayed in a smooth manner.
The present invention may further be arranged to perform a 3:2 pull-down display when a re-display of pictures is instructed by the repeat-first-field information. With this arrangement, a degradation of picture quality can be suppressed and a smooth display of pictures can be achieved even when an error stream to which the frame rate of 24 Hz has been set together with the repeat-first-field information is received.